PROJECT SUMMARY Autism Spectrum Disorders (ASD) are a collection of complex neurodevelopmental disorders characterized by repetitive behaviors and alterations in social interaction and communication that currently affect 1 in 68 children in the United States. Although it is clear that ASD prevalence is biased towards males (~4:1 male to female ratio), the mechanisms behind this male susceptibility remain elusive. While ASDs are quite heterogenous, evidence suggests a role for neuroinflammation in the development of these neurological abnormalities. Microglia, the resident immune cells of the brain, are key regulators of the neural response to immune activation and developmental organization of neural circuits, uniquely positioning them to translate early-life challenges into neural outcomes. Additionally, emerging evidence suggests that mitochondrial dysfunction plays an important role in neuropsychiatric/neurodevelopmental disorders such as ASD, and that inhibition of mitochondrial function may be critical in the processes of microglial activation. The emerging understanding of sex differences in mitochondrial and microglial physiology suggest that further study into their potential roles in the development of neurodevelopmental disorders such as ASD are warranted. To that end, I will first characterize the effect of early-life immune challenge (an established model for ASD in mice that presents with a strong sex bias) on mitochondrial gene/protein levels and functions. Building upon this, I will then assess how an early-life immune stimulus affects epigenetic regulation of mitochondrial DNA in males and females. Finally, I will ask what could be occurring during early development that may mediate these early- onset sex differences. There is a surge in gonadal hormone levels that occurs only in males during a ?critical period? of days surrounding birth that is responsible for masculinizing male brains. Injections of these hormones into females during this ?critical period? has been shown to ?masculinize? their brains to a male-like state. Using this model, I will determine if male sex hormones present during a critical period surrounding birth (?masculinizing? female mice) will impart susceptibility to subsequent immune challenge. I expect this project to result in several novel implications by bridging three bodies of research that are only beginning to collaborate: (i) the role of mitochondrial biology in neurodevelopment and disease, (ii) the role of neuroimmune dysfunction and microglial function in neurodevelopment and disease, and (iii) the striking differences in male and female responses to pathophysiological insults.